Clutch Disc and Brake Pad Friction

The science behind friction and what it means for you, your brakes, and your clutch

You can't escape friction. The force that resists motion between two objects that are in contact with one another exhibits itself all over the place: in between your engine's rod bearings and its crankshaft while spinning, for example. In between your brake's pads and its rotors while stopping. And even in between your bum and your seat, well, pretty much all the time.

It's friction that makes all sorts of important things happen. Sometimes you want less of it, which, in the case of an engine's rotating assembly, can result in a whole lot more power. Other times you want more, like the time you plowed the family minivan into your parents' garage door.

More than One Kind of Friction

Friction occurs on all surfaces, no matter how smooth they may seem. Microscopically, an engine bearing has a rough, jagged surface that when dragged across another surface, does all sorts of important things like break atomic bonds, put them back together, and create heat, which leads to slowing everything down.

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| Look around whatever it is that you drive and you'll find examples of friction happening all over the place, the most obvious of which is in between your brakes' rotors and its pads.

Science on smaller scales than we're accustomed to also tells us that there isn't just one type of friction. For starters, there's fluid friction, which has to do with the battle between multiple layers of a viscous fluid. There's lubricated friction, which has to do with fluid creating resistance between a couple of solid surfaces. There's skin friction, which has more to do with aerodynamics than anything else. There's internal friction, which has to do with the resistance between the elements that make up a solid material. And there's dry friction, which is what you should care about for the time being, and has to do with the resistance to lateral motion exhibited between a couple of solid surfaces.

Dig further and you'll realize that there's also more than one kind of dry friction: static, which describes what's going on in between non-moving surfaces, and kinetic, which does the same for moving surfaces, both of which can apply to brake pads or a clutch disc.

Friction and Your Brakes

There's no more obvious place on your car where friction manifests itself than in between your brake's rotors and its pads. It's what you rely on every time you step on that most important pedal. As it turns out, though, a whole lot happens in between you smashing that lever down and your tires coming to a halt.

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| Brake pads will determine just how well your car will stop despite whatever high-performance rotors or calipers you've got. But not all pads are created equal, and what's suitable for a track car, in most cases, won't perform nearly as well on the street where generating enough heat to make them work properly just isn't going to happen.

Your brake system starts with a pedal and ends at its pads, but there's a whole lot more in between. Step on the pedal and the brake master cylinder, which is located on the opposite side of the firewall and displaces hydraulic brake fluid from its reservoir throughout the system's lines that span to places like the ABS pump and all four calipers (or drums). It's that displaced fluid that allows each caliper's internal piston(s) to force its pads against its rotor, introducing friction and allowing your wheels to slow down or stop, depending on how much has been stirred up. (Without going beyond the scope of this article, now's a good time to mention that, technically, it isn't your brakes that stop your car; that would be your tires.)

Friction and Your Clutch

Friction's good for more than just slowing things down. Skimp on the stuff when it comes to something like a clutch and you're just wasting torque that could've made its way past your drivetrain. That's because it's friction that's responsible for your transmission's input shaft being able to soak up most of the energy your engine's trying to supply. It starts with your engine's flywheel, which is bolted directly to the southern end of its crankshaft and ends with your tires applying all of that torque to the pavement. In between all of that, though, is the clutch.

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| Every clutch assembly is made up of a flywheel, a pressure plate, and a disc(s). It's the disc that's sandwiched in between the two, which features a specialized friction lining on both sides and, in part, determines how well the clutch will engage and how much torque it'll all be able to hold.

Every clutch is made up of two major components: a pressure plate that's bolted directly to the flywheel and a disc, also called a driven plate or a friction lining, that's sandwiched in between the two that features friction surfaces on both sides—not unlike a brake pad—that make or break contact with the flywheel and pressure plate. Every pressure plate is made up of some sort of diaphragm that's able to clamp the disc in place, allowing the whole assembly to spin as a unit when your foot's off the pedal, or release pressure, disengaging the disc from the transmission and allowing you to shift when your foot's been taken away. All of that clamping and unclamping is made possible because of friction, and depending on what sort of disc you've got and the material its linings are made of, those clutch-engaging capabilities can range from fairly forgiving to so aggressive you might confuse your clutch for an on/off switch.

Coefficient of Friction

Knowing just a little bit of the science behind friction will, at best, help you end up with the right brake pads or clutch or, at worst, let you know how you ended up with the wrong stuff. It starts with the coefficient of friction, which is expressed by the Greek letter μ (pronounced mu), and demonstrates how well or how poorly friction exhibits itself between two objects. Practically speaking, a higher coefficient of friction means the better a pad or a disc will be able to do its job. Coefficient of friction values range from 0 (full lubricity) to 1.0 (solid). As far as brake pads go, for example, values as high as 0.60 can be found on many race-only applications, but don't be expecting your daily-driven Audi to need anything close to that.

Materials Matter

Brake pads and clutch disc linings can be made up of a number of different materials that'll affect all sorts of important things, like how well they'll work, how long they'll last, and how much heat they'll be able to deal with, for instance. And whether or not a particular material is used isn't just a crapshoot. Manufacturers have got to consider its coefficient of friction, whether or not it'll be affected by moisture, how well it'll wear, if it'll be capable of doing its job when things get really hot, and if it'll be able to resist high axial pressures. They've also got to be sure somebody just might actually want it, which is why finding something like a dual-diaphragm clutch with a full-faced, cast-iron disc for your 1982 Peugeot won't be likely.

Brake Pad Linings

Brake pads will determine how well or how poorly your car stops despite whatever rotors, calipers, or other brake components you've got. You ending up with the right pads starts with you being honest with yourself about what exactly it is you plan on doing with your car. Which means you slapping a set of race-only ceramic pads onto the same M3 you use for doing things like getting to work will never be a good idea. That's mostly because harder-compound pads that are suited for racing won't work as well on the street. As it turns out, the harder the pad, the longer it'll take to warm up, which means the chances of you plowing into your parents' garage door again just got a whole lot better. That's because a brake pad's ability to do its job isn't a constant; temperature, humidity, and wear can all affect how well it'll work this morning as opposed to tonight.

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| The number of different materials and compounds a brake pad can be made up of isn't small. Before choosing a pad, determine what exactly you'll be doing with your car and shop accordingly.

Non-metallic: These sort of pads can be made from all kinds of substances, including cellulose, sintered glass, and aramid, and then bonded together as a composite. As far as your brakes go, non-metallic pads won't last terribly long and can yield a fair amount of brake dust, but they won't scar up your rotors nearly as much as other materials might.

Semi-metallic: Here, various sintered metals are mixed with synthetic materials to form a set of pads that'll last longer than and resist fade better than non-metallic ones but will chew up your rotors a bit faster. Semi-metallic pads also have a lower coefficient of friction when compared to non-metallic ones, which means you'll have to press down on the brake pedal harder to yield the same amount of stopping power.

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| Often a brake pad's frictional capabilities will be classified on its backing plate. Here, performance values classified under SAE Standard J866 expressed as a two-letter code reveal how well it'll work under low-temperature (0 - 200 degrees F) and high-temperature (200 - 600 degrees F) conditions. If the first letter's lower than the second, the pad will be better suited to higher temperatures and has got to be heated up to work well. If the second letter's lower than the first, the pad might not work as well at higher temperatures. A good set of street pads that'll keep you from bumping into things you ought not to will feature similar friction characteristics at both low and high temperatures, which means both letters would be the same, like EPIC Friction's latest pads that are designed for all-around use.

Full-metallic: Unlike the others, fully metallic pads are made up of sintered metal—typically steel—and nothing else. They'll last longer than just about anything else and provide superior fade-resistant characteristics, but they won't do it quietly and they won't do it without eventually costing you your rotors.

Ceramic: Last are porcelain-and-clay-bonded pads that are typically joined with sintered copper or some other metal. Ceramic pads are a good compromise between how well metallic pads resist fade and how quiet non-metallic pads are, however, they aren't as good at dissipating heat.

Clutch Disc Linings

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| The same rules of friction apply no matter how many discs your clutch is made up of. Twin-disc setups like these feature a total of four friction surfaces—one for each side of both discs.

Like brake pads, you ending up with the right clutch has just as much to do with you being honest with yourself as it does what's available. That M3 you're still driving to work in? Clutch linings made up of harder materials and higher coefficients of friction won't be as easy to engage without a whole lot of chatter, which means all of a sudden your clutch will feel more like an on-off switch and you'll be one step closer to wishing you had an automatic.

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| Organic linings like these from SPEC are formulated using various soft metals embedded within synthetic fibers and resin. According to SPEC, organic discs like these are more stable and are capable of handling more torque than most OE discs yet offer the same sort of streetability and wear resistance.

Organic: The oldest and most common clutch linings, calling them organic doesn't make a whole lot of sense anymore. Organic clutches were historically made of asbestos—a good friction material because of its high resistance to heat and decent coefficient of friction but also pretty good at giving you cancer. Today, organic linings are made up of synthetic fibers and cellulose-type materials like cardboard and fiberglass as well as mineral wools that are embedded into a resin base and riveted to a disc. SPEC Clutch's organic linings, for example, are also formulated with various soft metals embedded within the synthetic fibers and resin. According to SPEC's David Norton, such materials are more stable and are capable of handling more torque than most OE discs, yet offer the same street driveability and wear resistance. Modern organic clutches do a good job at cushioning drivetrain components and make communicating with the pedal easy because of their smooth engagement but don't work terribly well when things get hot, like when a whole lot of torque has been applied. Here, their coefficients of friction can rapidly drop and, if things get hot enough, mechanically, they can glaze, wear, and fall apart.

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| Kevlar linings—not unlike what a bulletproof vest is made out of—are more aggressive and more heat resistant than an organic lining and feature a better burst strength, wear characteristics, and coefficient of friction when compared to organic linings.

Kevlar: Look for something marginally more aggressive and more heat resistant than an organic lining and you've found something made of Kevlar. Here, chopped fibers of the nylon-like material are bonded together, yielding something with a fair burst strength and wear characteristics superior to organic materials and with a marginally better coefficient of friction. Like an organic lining, a Kevlar one allows for smooth clutch engagement but requires higher clamping loads from the pressure plate in order to handle a significant amount of torque. "The only trade-off [is that Kevlar's] peak operating temperature is lower, so it takes longer to cool than organic," Norton says. "But within its torque capacity and without manual slippage, the operating temperature is not an issue." Kevlar discs can be as sensitive as they are tough, too. A poor break-in or even a small amount of stray grease left on the flywheel can significantly limit its life span.

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| Semi-metallic discs like this one from SPEC can handle a whole lot more than an organic or Kevlar disc can but can also be a bit less forgiving, resulting in a slightly more aggressive and sudden bite.

Semi-metallic: As with brake pads, semi-metallic clutch linings are made up of a variety of metals that can range from steel to iron to copper, along with some of the same materials you'd find in an organic lining. Compared to organic discs, semi-metallic ones also feature a woven structure but have higher coefficients of friction, can withstand a bit more heat, and are marginally more durable. Depending on the metallic content, semi-metallic discs can also be a bit less forgiving, resulting in a slightly more aggressive and sudden bite. SPEC's graphite, semi-metallic material is a good example of what the right mixture can yield, which in its case is better friction capabilities than anything organic, resistance to wear and noise, and the ability to remain fairly consistent despite higher temperatures. SPEC offers these materials in puck and full-faced configurations, the former of which won't engage as smoothly as an OE-style organic clutch but are certainly streetable. SPEC's full-faced configuration, however, offers the longest wear life out of any of its lineup, engages smoother than the competitors', according to Norton, and is quieter than anything made wholly of metal.

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| It's hard to beat a sintered-iron disc for the ultimate in grip. A sintered-iron disc's best quality is its ability to withstand slippage but without sacrificing its coefficient of friction, which makes it ideal for drag racing applications but not so ideal for street use.

Sintered iron: Sintered-metal linings are made up of powdered materials that have been poured into a mold and fused together using a whole lot of pressure and heat. A variety of metals can be sintered for use as a clutch lining, but iron is the most common. A sintered-iron disc's best quality is its ability to withstand slippage but without sacrificing its coefficient of friction, which is precisely what makes it ideal for drag racing applications where, under the right conditions, they work better as they get hotter. Clutch engagement is harsh here compared to organic, Kevlar or semi-metallic applications, but it's a necessary trade-off where friction can't be sacrificed. According to Norton, iron is SPEC's most aggressive material and has a higher coefficient of friction than anything else in its line. "Iron discs have tremendous heat control," he says, "and will slip into full lock as surface temperatures increase." The only downside, he explains, is that iron discs are heavy, can chirp or squeak when engaged, and can create a high rate of surface wear.

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| The latest breakthrough in clutch-lining technology is the carbon-carbon disc. Here, every friction surface from the flywheel to the disc is covered in carbon. Only carbon-carbon discs offer the best combination of high torque capacities, quick shifting capabilities, soft driveline engagement, and great manageability. They can also be terribly expensive.

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Carbon-carbon: Cover every friction surface within the clutch and flywheel assembly with amorphous carbon (which means it's not stranded like your carbon-fiber hood is) and you've got one of the most recent developments in clutch-lining technology—the carbon-carbon disc. Carbon linings like these are subjected to specialized cycles where they're introduced to enormous amounts of heat before being machined into shape. Along the way, friction modifiers can also be integrated, which can make a carbon-carbon disc not just one of the lightest options available but also one of the most effective, despite whatever heat it's subjected to. "Carbon-carbon is the pinnacle of friction technology," Norton says, and goes on to explain how carbon-carbon discs, like those in SPEC's Super Carbon series, feature high-torque capacities, lightning-fast shifting capabilities, soft driveline engagement, and great manageability. And according to Norton, the material won't break down this side of 5,000 F. It'll also last longer than just about any other disc material.

According to Norton, there are all sorts of considerations you've got to make before settling on the right clutch or, for that matter, a set of brake pads. Torque capacity, drivability, life expectancy, weight, and cost are all characteristics that'll determine whether or not that clutch, for example, is right for you, and all of which have got at least a little something to do with friction.